1. Field of the Invention
This invention relates to an adhesive composition suitable for a hot melt adhesive or a hot melt type pressure-sensitive adhesive. The adhesives composition of the present invention exhibits excellent properties such as process stability, adhesion, creep resistance, flexibility at low temperature and solvent resistance so that it can be used in a variety of applications such as, for example, packaging, bookbinding, plywood construction, woodworking, making shoes, adhesion of textile goods and production of various adhesive goods such as various kinds of adhesive tapes and labels.
2. Description of the Background
Solvent type adhesives have been widely used in various applications. The solvent type adhesive has, however, the problems of air pollution, danger of a fire, worsening of the labor environment and industrial hygiene because it usually contains an organic solvent as the solvent. Recently, therefore, solvent-free adhesives have come to be used widely as a substitute for solvent type adhesives. Examples of solvent-free adhesives include aqueous adhesives and hot melt adhesives. Of these, hot melt adhesives are preferably used because they require no drying time or no drying apparatus, and provide excellent energy savings, as well as working properties and productivity.
Also, pressure-sensitive adhesives as a type of adhesive are known. Adhesive goods which have a layer of a pressure-sensitive applied to a base material such as tape, film or sheet of paper, cloth or plastic are used in a wide variety of applications as adhesive tapes, adhesive films and adhesive sheets. In the production of such adhesive goods having a pressure-sensitive adhesive layer, it is common practice to apply a solution of the pressure-sensitive adhesive in an organic solvent to a base material, and then remove the organic solvent, to form the pressure-sensitive adhesive layer on the base material. However, pressure-sensitive adhesives also present problems of air pollution, danger of fire and worsening of the labor environment and industrial hygiene, because of the use of an organic solvent. Under such circumstances, a solvent-free pressure-sensitive adhesive has been required from the viewpoints of safety, working properties and energy saving. To meet the above requirements, a hot melt pressure-sensitive adhesive, which can be applied to a base material without an organic solvent, has been developed.
For the hot melt adhesive and hot melt pressure-sensitive adhesive, a thermoplastic polymer, which can be melted by heating, is used as the base polymer. Examples of the base polymer used for the hot melt adhesive include ethylene-vinyl acetate copolymers, ethylene-.alpha.-olefin copolymers, polyamides, polyesters and block copolymers comprising the polymer block of an aromatic vinyl compound and the polymer block of a conjugated diene such as styrene-butadiene-styrene block conjugated diene such as styrene-butadiene-styrene block copolymer or styrene-isoprene-styrene block copolymer. Examples of the base polymer used for the hot, melt pressure-sensitive adhesive include ethylene-vinyl acetate copolymers, block copolymers comprising the polymer block of an aromatic vinyl compound and the polymer block of a conjugated diene, and acrylate-based polymers.
Among the above-exemplified base polymers, an ethylene-vinyl acetate copolymer is used widely as a base polymer for hot melt adhesives or hot melt pressure-sensitive adhesive, because of its toughness, its good adhesion to various base materials such as paper, wood, plastics and metal, its good compatibility with other components (tackifier, wax, softening agent or the like) and its excellent hot melt flowability and thermal stability. However, the ethylene-vinylacetate copolymer has the problem that, because of its insufficient flexibility at low temperatures, the products which use a hot melt adhesive employing the ethylene-vinyl acetate copolymer as the base polymer or adhesive goods having a layer of a hot melt pressure-sensitive adhesive employing the ethylene-vinyl acetate copolymer as a base polymer are not suited for use at low temperatures.
Compared with the ethylene-vinyl acetate copolymer, polyamides and polyesters have lower compatibility with other components of an adhesive such as tackifiers, and, moreover, exhibit inferior handling properties and adhesion to various base materials.
A block copolymer, which is not hydrogenated, comprising the polymer block of an aromatic vinyl compound and the polymer block of a conjugated diene, on the other hand, has been used widely as a base polymer for a hot melt adhesive or hot melt pressure-sensitive adhesive because of its comparatively good adhesion and tackiness at room temperature. Such a block copolymer is, however, considerably inferior in its heat aging resistance and weatherability, because it contains an unsaturated carbon--carbon double bond derived from a conjugated diene in its molecule. A hydrogenated product of the block copolymer comprising the polymer block of an aromatic vinyl compound and the polymer block of a conjugated diene is sometimes used for a hot melt adhesive or hot melt pressure-sensitive adhesive as a base polymer with improved heat aging resistance and weatherability. Since such a hydrogenated block copolymer product cannot be applied to a base material at low temperature because of its high melt viscosity, it is difficult to apply the hydrogenated product to a base material having a low heat deformation temperature such as a polyethylene film or unwoven fabric made of polyethylene. In addition, the melt viscosity of the hydrogenated product changes significantly with only slight changes in temperature. Thus, the hydrogenated product possesses inferior process stability and is significantly limited in its processing conditions for the preparation of an adhesive composition comprising the hydrogenated product.